Historically, airbags have been coated with one or more layers of polymeric material to enhance their performance, for example, by preventing the unwanted permeation of air through the fabric and, to a lesser extent, by protecting the fabric from detriment due to exposure to hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of airbags. However, it was subsequently discovered that, when exposed to heat, polychloroprene tends to degrade and to release the components of hydrochloric acid, thereby potentially introducing hazardous chemicals into the surroundings and degrading the fabric component. This degradation issue, coupled with the desire to decrease the folded size of the completed airbag by using less coating material, has led to the almost universal replacement of polychloroprene with silicone-based materials for use as airbag coatings.
Newer designs for airbags, particularly those being placed in the sides of passenger compartments, have introduced the requirement that the bags hold pressure longer under use. The requirement of longer air retention and the use of the lower coating levels of silicone polymer have begun to highlight the effect that a naturally lubricating silicone coating may allow the yarns from which the fabric is constructed to shift when a sewn seam is put under stress. This shifting can lead to leakage of the inflating gas through the pores formed from the shifting yarns or, in drastic cases, can cause the seam to fail. Since the airbag must retain its integrity during a collision event in order to sufficiently protect the driver or passenger, there is a great need to provide coatings that provide both effective air retention characteristics and sufficient restriction of yarn shifting for the airbag to function properly.
As mentioned above, in recent years, silicone coatings have been utilized to provide such desired permeability and strength characteristics. However, the relative cost of such coating materials (such as polydimethylsiloxane) is sufficiently high that new, more inexpensive alternatives are being sought. Thus, there exists a need for providing good adhesion and a strong bond between the individual yarns, in order to effectuate long-term rigidity of the fabric to prevent unraveling at cut edges or at seams, while simultaneously providing aging stability and excellent low air permeability characteristics.
To provide a cost-effective and functional replacement for coatings containing only silicone, multi-layer coating systems have been developed. Materials used to create such coatings include polymers such as polyurethane, acrylics, and the like, which are used either alone or in combination with silicone.
For example, U.S. Pat. Nos. 6,239,046 and 6,641,686, both to Veiga et al., describe the use of a two-layer airbag coating, where the fabric-contacting layer is an adhesive polyurethane and the top layer is an elastomeric polysiloxane. Another approach, described in U.S. Pat. No. 6,734,123 to Veiga et al., uses multiple layers of polyurethane as the airbag coating material. In this instance, layers of adhesive polyurethane and elastomeric polyurethane are employed to achieve the desired properties. Yet another multi-layer coating system is provided in U.S. Pat. No. 6,770,578 to Veiga, in which a prime coat of polyurethane is applied to an airbag fabric, followed by one or more layers of polymer film. Such polymer films are formed of polyurethane, polyamide, or polyolefin. None of these references, however, teach a hybrid resin used as an airbag coating.
Other efforts to create multi-component airbag coatings have focused on combining silicone with different polymers in the same polymer network. U.S. Pat. Nos. 6,348,543; 6,468,929; and 6,545,092, all to Parker, describe the production of an airbag coating made of a vinyl-containing polysiloxane cross-linked to, or admixed with, an ethylene-containing copolymer, such as ethylene methyl acrylate or ethylene vinyl acetate. In an alternate approach, described in U.S. Pat. No. 6,846,004 to Parker, a silicone polymer is combined with a copolymer of ethylene and at least one polar monomer in the presence of a volatile solvent and, optionally, a curing catalyst. Yet another approach, which is described in US Patent Application Publication No. 2005-0100692 to Parker, involves coating the airbag fabrics with the cross-linked reaction product of a vinyl-containing silicone and a copolymer having silicone and non-silicone substituents, which may or may not have terminal Si—H groups.
One issue with these systems is the difficulty of forming a uniform blend of the selected polymers. Moreover, a simple blend of polymers does not result in the molecular “interlacing” that is characteristic of the presently desired IPN, because most polymer resins are not compatible with each other. When different resins are simply blended together, they have a tendency to form adjacent, but separate, domains. Most often, these polymers have been polymerized individually prior to being combined with another polymer. Each resin component forms its own domain in the blend, resulting in areas of the coating with different physical properties (such as melting point or flash temperature). When considered as a whole, the blend typically exhibits properties that are a compromise of the bulk properties of the individual resins. The interfaces between two different resin domains are the weak points of the blend, which may contribute to poor mechanical properties and poor long-term stability of the resin blend.
To overcome the issues with simple blends of polymers, hybrid polymer resins have been developed. Hybrid polymer resins are mixtures of at least two different polymer resins, which are combined in such a way as to form an interpenetrating polymer network (IPN). Such interpenetrating networks result from the two resin materials interlocking with each other on a molecular level, thus providing superior properties (such as abrasion and toughness) as compared to a corresponding simple blend made without forming an IPN structure. Often, these hybrid resins include resins having different physical or chemical properties, such as flame retardance, that are desired when the hybrid resin is used as a coating for textile substrates.
IPNs can be produced using a number of different methods. They can be formed by dissolving one monomer into another polymer resin, followed by polymerization of the monomer in the other polymer matrix. IPNs can also be made by sequential or simultaneous polymerization of an intimate mixture of two or more different monomers. One can also mix two pre-polymers to form an intimate mix and post-polymerize the pre-polymers such that the final polymers form an IPN structure. Finally, one can mix two polymers together, either in melt or in solution, to form an intimate mixture and subsequently cross-link at least one of the polymers.